The present invention relates to railroad freight cars, and in particular to the structure of a car intended to carry intermodal freight containers in a container well.
Railroad cars have been used to carry intermodal freight containers for decades. Many such cars can carry a pair of short containers end-to-end in a container well defined by a pair of spaced-apart side sills, as shown, for example, in Hill, et al., U.S. Pat. Nos. 4,893,567, 5,054,403, and 5,170,718; Hill, U.S. Pat. Nos. 4,599,949 and 4,703,699; Tylisz, et al., U.S. Pat. No. 5,085,152; Zaerr, et al., U.S. Pat. No. 6,510,800; and Smith, et al., U.S. Pat. No. 6,546,878. While such cars must be built sufficiently strong to support the weight of loaded containers, as well as being capable of sustaining the forces imposed on a car during operation of a railroad train, it is desirable to minimize the weight of each car itself, so that it can carry a greater weight of revenue producing laden containers without exceeding the maximum weight permitted to be imposed on a railroad track.
In such container well cars side sills serve both as side walls of a container well and to carry the many dynamic forces imposed by movement of the car as part of a train. The side sills also have to carry the bending loads resulting from the weight of containers carried in the well or stacked atop a container or a pair of containers carried in the well. A pair of short containers carried end-to-end in the well impose part of their weight on the side sills near the middle of the length of the container well. The side sills must thus be able to sustain the weight of the adjacent ends of a pair of short containers located in the middle of the length of the container well.
Because of the overall size limitations within which a loaded railway car must fit, as a result of the clearances along a railway and the configuration of conventional side-loading equipment available for loading containers onto railroad cars, only a limited space is available within which the side sill structures of a container-carrying railroad freight car may be constructed. Nevertheless, the side sills must have sufficient strength to support the vertical beam loads applied when the car is laden, and to resist torsional and axial stresses resulting from the loads applied during travel of a laden car, while the weight of the side sill structures should be kept to a minimum consistent with the required strength.
Since cargo containers are placed between the side sills of a well car, structural interconnection between the top edges of the side sills is prevented, and each side sill must have sufficient torsional rigidity to prevent structural failure when such a well car is laden. This is particularly true when two shorter containers, such as 20-foot containers, are carried end-to-end in a container well of such a car, applying substantial vertical loading midway between the supporting trucks of the car.